1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method. More specifically, suppressing a decrease in throughput by controlling stage driving.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate a micropatterned semiconductor device such as a semiconductor memory or logic circuit by using photolithography. The projection exposure apparatus projects and transfers, by a projection optical system, a pattern (circuit pattern) formed on a reticle (mask) onto a substrate (e.g., a wafer) coated with a photosensitive agent such as a resist.
An exposure apparatus of the step & scan scheme (called a “scanner”) has become the current mainstream in place of an exposure apparatus of the step & repeat scheme (called a “stepper”). The step & scan scheme means an exposure scheme of transferring the pattern of a reticle to a given shot region on a wafer while scanning the reticle and the wafer, and moving the wafer step by step to move the exposure target position to the next shot region after the end of exposure of the given shot region.
A resist generally has an exposure amount (to be referred to as a “set exposure amount” hereinafter) D (J/m2) specified to transfer (form) the pattern of a reticle onto the resist with high accuracy. For this reason, a scanning velocity V (mm/sec) of a wafer (a stage which holds the wafer) in a scanner needs to satisfy:V≦Imax/D×Ws  (1)where Imax (W/m2) is the maximum illuminance of the exposure light on the wafer, and Ws (mm) is the exposure slit width in the non-scanning direction on the wafer.
Referring to relation (1), a maximum scanning velocity Vd determined from the set exposure amount D is given by:Vd=Imax/D×Ws  (2)
Moreover, since a scanner has a maximum scanning velocity Vmax which is virtually determined based on a stage control system, including specifications associated with its structural and mechanical performances, the scanning velocity V needs to satisfy:V≦Vmax  (3)
A scanner synchronously controls a reticle and a wafer so that they hold a predetermined positional relationship, and transfers (forms) the pattern of the reticle onto the wafer by scanning the reticle and the wafer. However, when the reticle and the wafer suffer a deviation (to be referred to as a “synchronization error” hereinafter) from the predetermined positional relationship, this leads to a decrease in resolution and a misalignment in imaging position of the pattern and, in turn, disturbs the manufacture of semiconductor devices. The synchronization error is nearly proportional to the scanning velocity; the synchronization error increases as the scanning velocity rises. Accordingly, the maximum scanning velocity Vmax is determined to keep the synchronization error within a tolerance.
It is also necessary to expose the wafer with a plurality of pulsed light beams including pulses the number of which is equal to or larger than a predetermined pulse count (to be referred to as a “minimum pulse count” hereinafter) P when pulsed light such as a KrF excimer laser beam or an ArF excimer laser beam is used as the exposure light. Meeting this requirement makes it possible to uniform the integrated exposure amount while suppressing the influence of a variation in energy per pulse of the pulsed light. Hence, the minimum pulse count Pmin needs to satisfy:Pmin≦Ws/V×f  (4)where f (Hz) is the oscillation frequency of the exposure light (pulsed light).
Referring to relation (4), a maximum scanning velocity Vp determined from the minimum pulse count Pmin is given by:Vp=Ws/Pmin×fmax  (5)where fmax is the maximum oscillation frequency of the exposure light.
Under the circumstances, the oscillation frequency of the exposure light is determined to be the maximum oscillation frequency fmax for a resist having a relatively large set exposure amount D, and the scanning velocity is determined to be the maximum scanning velocity Vmax for a resist having a relatively small set exposure amount D, so as to satisfy relations (1), (3), and (4) in both cases.
For example, a case in which the integrated exposure amount can be equal to the minimum pulse count Pmin regardless of the maximum scanning velocity Vd (equation (2)) determined from the set exposure amount D, the maximum scanning velocity Vmax determined from the stage control system, and the set exposure amount D will be considered. In this case, the minimum value of the maximum scanning velocity Vp (equation (5)) determined from the minimum pulse count Pmin is determined as the scanning velocity during actual exposure.
To improve the throughput, a scanner typically sequentially transfers the pattern of a reticle to a plurality of shot regions on a wafer by alternate scanning (reciprocating scanning) of the reticle. This requires an operation (over-scanning) for further moving the reticle by the same distance as the moving distance upon pre-scanning before the start of exposure of the next shot region after the end of exposure of one shot region, thereby returning the reticle to the scanning start position to expose the next shot region. Also, an operation for moving the wafer in the scanning direction is needed, in addition to an operation for moving the wafer to the next shot region (another shot region adjacent to one region in the non-scanning direction) step by step. Note that the moving distance upon pre-scanning means the moving distance in the acceleration time for which the stage accelerates until its velocity reaches a target velocity (the scanning velocity during exposure), and in the settling time taken for the stage to settle from when its acceleration ends until its velocity reaches a target velocity (falls within a tolerance) (i.e., until stage vibration dies down).
More specifically, the procedure of an operation for moving the wafer between shot regions is as follows:
(1) the wafer (the stage which holds the wafer) is moved to the same coordinate position in the scanning direction as the scanning start position of the next shot region after the end of exposure of one shot region;
(2) the wafer (the stage which holds the wafer) is moved to the scanning start position of the next shot region step by step in the non-scanning direction; and
(3) scanning of the wafer (the stage which holds the wafer) is started in order to expose the next shot region.
In this manner, the wafer moves along a roughly U-shaped route. During this time, that is, during the time from when exposure of one shot region ends until exposure of the next shot region starts (until stage deceleration in the scanning direction starts), setting information such as control parameters necessary to expose the next shot region is sent to the stage control system. This allows stage control (stage driving) so as to prevent the stage from stopping during the time from when exposure of one shot region ends until the settling time taken for exposure of the next shot region to be ready comes, thus improving the throughput. However, to attain stage control so as to prevent the stage from stopping during that time, movement of the wafer (the stage which holds the wafer) in the non-scanning direction needs to be completed before the completion of wafer movement in the scanning direction. Japanese Patent Laid-Open No. 2004-072076 discloses details of this technique.
Unfortunately, depending on the conditions such as the scanning velocity and the moving distance (stepping distance) of the wafer (the stage which holds the wafer) in the non-scanning direction, wafer movement in the non-scanning direction often cannot be completed before the completion of wafer movement in the scanning direction. In such a case, it is impossible to attain stage control so as to prevent the stage from stopping during the foregoing time. This leads to an unsatisfactory improvement in throughput and even causes a decrease in throughput.